Implantable devices are used for a variety of surgical procedures. One such device is a stent. Stents are generally cylindrical shaped devices that are radially expandable to hold open a segment of a blood vessel or other anatomical lumen after implantation into the body lumen to relieve intraluminal constrictions caused by disease or tissue trauma. Although stents alone have been successful in relieving constrictions, these constrictions or blockages reoccur in many cases. This reoccurrence is called restenosis and is due to the body's immune system responding to the trauma of the surgical procedure.
To reduce restenosis, stents have been developed with coatings to deliver drugs or other therapeutic solutions. Once the stent is positioned in a target site, these coatings offer long-term treatment from the drug by a controlled release of a specific amount of the drug from the surface of the stent. The rate of release depends upon the chemical and or biological composition of the drug and the amount of the drug depends upon the total depth and depth consistency of the drug coating layer on the stent surface. It has been discovered that methods of loading drugs onto implantable devices may be deficient in their current drug-loading and drug-delivery characteristics. In particular the amount or volume of the drug capable of being delivered to the target site may be insufficient due to the limited surface areas on the stent and the control of the rate of elution is limited by the chemical characteristics of the drug. In addition, during delivery of the stent, any coating exposed to the body lumen can lose a portion of the coating during delivery, either as a result of bloodflow over the surface, or by contacting the vessel tissue prior to delivery to the target site.
To increase the amount of the drug that may be deposited on the surface of the stent, the surface of the stent framework has been modified. Such modifications may be the formation of openings in the stent surface to hold more of the drug. For example, depots can be formed into the surface of the stent with the use of a multistep chemical etch process or with the use of lasers. However, since the extra amount of drug that can be held in the depots depends directly upon the depth of the depots and an increase in the depth of the depot will reduce the strength and the resiliency of the stent, only a limited extra amount of drug can be placed in the depots. Also the control of the rate of elution is still limited to the chemical characteristics of the drug.
Various types of stents are currently in use. FIGS. 1-4 illustrate stent portions in common use. Each of the illustrated stents suffer from undesirably limited surface area on the stent for the placement of a drug coating or for the placement of openings in the narrow remaining areas of the stents called struts. Also, removal of a portion of stent struts, such as to form openings in the surface to contain an additional amount of the drug, can undesirably limit the strength or resiliency of the stent being so machined. Each of the stents in FIGS. 1-4 have two channels 50 formed in struts 52 made in accordance with this invention into which a drug can be loaded. FIG. 1 illustrates a stent 10 stamped from a flat sheet of metal having ends 12, 14, 16, and 18. FIG. 2 shows the stamped flat sheet after being bent into a cylinder with ends 12 and 14 and 16 and 18 respectively being joined to hold the flat sheet in a cylindrical shape. FIG. 3 shows a stent 20 formed by joining a plurality of stamped rings 22 in which rings are bent into a sinusoidal form. Some of the points 24, which are defined as a peak deviation from the centerline of the sinusoidal ring, are joined together to fasten segments 22 which form the stent. Finally, stent 30, shown in FIG. 4, is formed from a wire 32 or a plurality of wires 32 that are bent to form the stent. The surface area onto which the drug can be loaded and each strut 52 is limited so that a channel with a very large depth will reduce the strength or resiliency of the stent.
Formation of surface modifications can reduce structural integrity due to loss of material during manufacture. It is desirable, in certain applications, to increase structural integrity. In addition, handling and packaging of drug loaded stents is complicated by the presence of the drug on the stent. For example, frictional contact with a surface, such as during packaging or shipping, can abrade at least a portion of the drug from the stent surface. Furthermore, certain treatments are better obtained by reducing elution of the drug from the stent during traversal of vasculature.
It would be desirable, therefore, to advance the art.